Run ShellCheck on scripts in GitHub Actions workflows

[samestep]

Currently we run ShellCheck in GitHub Actions on all our *.sh scripts in .jenkins/pytorch. However, it'd be nice to also check the shell scripts embedded in our .github/workflows/*.yml files.

Strictly speaking, we can't just pull out the contents of run: blocks and run ShellCheck on them, because

1. they might contain GitHub Actions expressions ${{ <expression> }}
2. they might use a different shell besides bash

However, judging from #55157, it seems like we could/should use a preprocessing step that simply forbids the first situation, and use a bit of logic (based on whether the platform is Windows, and the presence of the shell key) to resolve the second.